This invention relates to security articles comprising a hologram.
Holograms have been commonly used as authenticating or security devices because they have been considered difficult to manufacture and to reproduce or copy. Holograms are records of an interference pattern formed by light at a recording location and can be manufactured by first illuminating an object with a coherent light beam, e.g., a laser beam. Light reflected from the object is allowed to strike a light sensitive recording medium such as a photographic film or plate, and, simultaneously, a reference beam (comprising a portion of the light from the light source) is directed so as to bypass the object and to strike the recording medium directly. (Thus, the light incident on the recording medium is the sum of the light reflected from the object and the reference beam.) When the recording medium is later illuminated by coherent light (i.e., a laser beam or sunlight), an observer sees a virtual image of the object, which appears to exist in 3D space. Some types of holograms are visible even in non-coherent light (e.g., diffused light).
Holograms have commonly been incorporated into credit cards and the like as anti-fraud devices, with the idea that forgers do not have the requisite sophisticated equipment for manufacturing the holograms. However, forgers have developed counterfeiting methods. One response to the increased prevalence of counterfeiting has been to produce holograms of increasing complexity, but this has also led to increased cost. Other approaches have relied upon the use of covert images and special authentication or verification equipment, e.g., a laser, to enable the detection of such images, but such equipment has often been expensive and difficult to use. Thus, there is a continuing need in the art for secure holographic articles that are extremely difficult to counterfeit, that can be cost-effectively produced, and that can be easily and inexpensively authenticated or verified under field conditions.
Briefly, in one aspect, this invention provides a verifiable holographic article comprising a substrate comprising (a) at least one hologram; and (b) at least one covert image that is convertible to an overt image using a decoding sampling device, e.g., a photocopier, that produces an aliasing or moire effect. Preferably, the covert image is a high frequency image that is substantially imperceptible to the human eye, and that is created by first removing the frequency components of an original, overt image that are greater than half the sampling frequency of the decoding sampling device and then mirroring the remaining frequency components about the axes that correspond to half of the sampling frequency.
The verifiable holographic article of the invention cost-effectively provides both xe2x80x9cfirst linexe2x80x9d (hologram) and xe2x80x9csecond linexe2x80x9d (covert image) security features, thereby increasing the difficulty of counterfeiting (which requires sophisticated knowledge in fields beyond holography) and improving the level of security of the holographic article. Yet the article can be easily, quickly, and safely verified or authenticated using a photocopier or other simple sampling device, without the need for, e.g., expensive, difficult to use, and potentially dangerous laser detection equipment.
In other aspects, this invention also provides a security label or security laminate comprising the verifiable holographic article of the invention and a kit comprising the article and a decoding sampling device.